Late Quaternary micromammals and the precipitation history of the southern Cape, South Africa: response to comments by F. Thackeray, Quaternary Research 95, 154–156

HAL Id: hal-02931832

https://hal.umontpellier.fr/hal-02931832

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Late Quaternary micromammals and the precipitation

history of the southern Cape, South Africa: response to

comments by F. Thackeray, Quaternary Research 95,

154–156

J. Tyler Faith, Brian M. Chase, D. Margaret Avery

To cite this version:

J. Tyler Faith, Brian M. Chase, D. Margaret Avery. Late Quaternary micromammals and the precipi-tation history of the southern Cape, South Africa: response to comments by F. Thackeray, Quaternary Research 95, 154–156. Quaternary Research, Elsevier, 2020, 95, pp.157-159. �10.1017/qua.2020.11�. �hal-02931832�

Late Quaternary micromammals and the precipitation history of the southern

1

Cape, South Africa – Response to comments by F. Thackeray, Quaternary

2

Research XX(X), pp. XXX-XXX

3 4 5

The authors do not recommend the distribution of this version

6

of this article.

7

The article is freely available upon request.

8

To receive a copy, please send a request to Brian Chase at:

9

brian.chase@umontpellier.fr

J. Tyler Faitha,b*, Brian M. Chasec, D. Margaret Averyd 14

15

a

Natural History Museum of Utah, University of Utah, 301 Wakara Way, Salt Lake City, 16

UT 84108 USA 17

18

b

Department of Anthropology, University of Utah, 260 S. Central Campus Drive, Salt 19

Lake City, UT 84112 USA 20

21

c

Institut des Sciences de l'Evolution-Montpellier (ISEM), University of Montpellier, 22

Centre National de la Recherche Scientifique (CNRS), EPHE, IRD, Montpellier, France 23

24

d

Iziko South African Museum, P.O. Box 61, Cape Town, 8000 South Africa 25

26

INTRODUCTION

27

We appreciate Thackeray’s (2020) comments on our recent examination of late 28

Quaternary micromammals from the southern Cape of South Africa (Faith et al., 2019). 29

Focusing on the well-sampled sequence from Boomplaas Cave, we argued— 30

controversially in Thackeray’s (2020) opinion—that the micromammals indicated a 31

transition from a relatively humid Last Glacial Maximum (LGM) to a more arid Holocene. 32

This is at odds with earlier interpretations of the region’s climate history (e.g., Avery, 33

1982; Deacon et al., 1984; Deacon and Lancaster, 1988), though it is now supported by 34

a growing body of evidence (e.g., Chase et al., 2017; Chase et al., 2018; Engelbrecht et 35

al., 2019; Faith, 2013a, b). We welcome this opportunity to clarify a few points raised by 36

Thackeray (2020) and to further elaborate on our original interpretations. 37

38

MOISTURE AVAILABILITY, PRECIPITATION, AND TEMPERATURE

In Faith et al. (2019), our analysis and interpretation focused specifically on moisture 40

availability (humidity/aridity). As defined in the paper, this variable is determined by 41

precipitation minus potential evapotranspiration. While it is common to conflate aridity 42

with rainfall amount, as Thackeray has in his comment (2020), this leads to confusion, 43

as rainfall amount is only one factor determining aridity. As discussed by Chevalier and 44

Chase (2016), moisture availability is largely determined by the combination of 45

precipitation and temperature, through its influence on evapotranspiration. Thus, our 46

interpretation of relatively humid conditions during the LGM at Boomplaas Cave should 47

not be equated as implying relatively higher rainfall, as Thackeray (2020) has inferred. 48

To be clear, a relatively humid LGM could result from greater precipitation, cooler 49

temperatures, or a combination of both. There is no question that cooler temperatures 50

during the LGM would have contributed to greater moisture availability by reducing 51

evapotranspiration (as suggested by Chase et al., 2017; Chase et al., 2018), but 52

whether this was accompanied by higher or lower precipitation cannot be ascertained 53

from our analysis. Indeed, we are skeptical that any analysis of faunal community 54

composition can inform directly on rainfall amount sensu stricto, when it is moisture 55

availability that determines habitat structure and the availability of the key resources 56

(e.g., forage, standing water) on which faunas depend (Faith and Lyman, 2019). Faith et 57

al., 2019 focused on moisture availability precisely because most organisms (both floral 58

and faunal) are influenced by moisture availability rather than by rainfall amount—as a 59

given amount of precipitation can have vastly different environmental consequences 60

depending on how much of it is lost through evapotranspiration (e.g., Chevalier and 61 Chase, 2016). 62 63 A SEMI-ARID CLIMATE 64

Thackeray (2020) observes that in our ordination of modern and fossil micromammal 65

samples, the LGM assemblage from member GWA at Boomplaas Cave plots adjacent 66

to several modern assemblages characterized by a semi-arid climate. The emphasis 67

Thackeray (2020) places on ‘semi-arid’ throughout his letter implies that some 68

clarification is necessary, since the implication is that a semi-arid LGM is inconsistent 69

with our original interpretations. It is not. Following the United Nations National 70

Environment Programme (UNEP) classification scheme (UNEP, 1997), a “semi-arid” 71

climate is characterized by mean annual precipitation (MAP) that is 20-50% of mean 72

annual evapotranspiration (MAE), or, aridity index (AI; MAP/MAE) values of 0.2 to 0.5. 73

Boomplaas Cave today is at the lower limit of semi-arid (AI = 0.24), yet the modern 74

samples flagged by Thackeray (2020) are characterized by much greater moisture 75

availability, with AI values of 0.43 to 0.48 (i.e., MAP is 43-48% of MAE). In other words, 76

GWA is similar to modern micromammal assemblages from places with nearly double 77

the moisture availability of the Boomplaas environment today, but which are also 78

classified as “semi-arid”. Thus, the proximity of GWA to these assemblages is fully 79

consistent with our previous observation of a relatively humid – but still semi-arid - LGM. 80

81

CUTTING THROUGH THE CONFUSION

82

Resolving the paleoclimatic history of the southern Cape has proven challenging in part 83

due to a combination of seemingly contradictory lines of evidence (e.g., Avery, 1982; 84

Avery, 2004) together with conflicting interpretations of the evidence (e.g., Chase and 85

Meadows, 2007; Deacon et al., 1984; Faith, 2013a; Faith et al., 2019; Thackeray, 86

2020). These conflicts arise because many of the key archives provide only indirect— 87

and at times uncertain—proxies for the climate variables in question. Indeed, many 88

characterizations of the LGM as a time of harsh and arid conditions are based on 89

ambiguous evidence (reviewed in Chase and Meadows, 2007), and this is particularly 90

true of the records from Boomplaas Cave (see discussion in Faith, 2013a). For 91

example, focusing on the micromammals, Avery (1982) once argued that low taxonomic 92

diversity during the LGM was indicative of arid conditions, though she later showed that 93

diversity was a poor predictor of precipitation (Avery, 1999). Thackeray’s (1987) 94

interpretation of an arid LGM was based on a micromammal-derived index that is only 95

weakly correlation with precipitation (r2 = 0.35), implying that it is strongly influenced by 96

other (currently unknown) environmental parameters. Likewise, elevated frequencies of 97

the bush Karoo rat (Myotomys unisulcatus) during the LGM has also been interpreted 98

as indicative of aridity (Avery, 1982; Deacon et al., 1984; Thackeray, 1987)—most 99

recently by Thackeray (2020)—yet this species occurs at similar if not higher 100

abundances in environments that are considerably more humid than Boomplaas Cave 101

is today (Supplementary Table 1 in Faith et al., 2019). 102

Because the reconstruction of paleoclimatic changes from the mammalian fossil 103

record is fraught with potential pitfalls, confidence in the interpretations is enhanced 104

when there is consistency between multiple independent lines of evidence (Faith and 105

Lyman, 2019). Our interpretations provide just that. In Faith et al. (2019), we 106

emphasized the broad similarities between our record of moisture availability and that 107

provided by isotopic analysis of the Seweweeksport hyrax middens (Chase et al., 2017; 108

Chase et al., 2018), located in a similar environment ~70 km west of Boomplaas. Also 109

important is that the nearby Cango Cave speleothem (~3 km east of Boomplaas) shows 110

a hiatus from the Lateglacial to the middle Holocene, signaling a lack of drip water 111

availability (Talma and Vogel, 1992; Vogel, 1983). Deacon et al. (1984) struggled to 112

reconcile this with their interpretations of an arid LGM transitioning to a humid 113

Holocene, though the timing of the hiatus closely matches what we infer to be the most 114

arid portion of the Boomplaas Cave sequence (Faith et al., 2019). In addition, a recent 115

climate simulation suggests that the region would have received greater rainfall during 116

the LGM relative to the present (Engelbrecht et al., 2019). In our view, the consistency 117

between all of these records tips the scale in favor of the emerging understanding of the 118

southern Cape’s climate history—the transition from the LGM to the Holocene was 119

characterized by increased aridity. 120

121

ACKNOWLEDGMENTS

122

Though we may not be in agreement on this issue, we thank Francis Thackeray for his 123

collegiality and for his efforts to better understand the late Quaternary climates of South 124 Africa. 125 126 REFERENCES 127

Avery, D.M., 1982. Micromammals as palaeoenvironmental indicators and an 128

interpretation of the late Quaternary in the southern Cape Province, South Africa. 129

Annals of the South African Museum 85, 183-374. 130

Avery, D.M., 1999. A preliminary assessment of the relationship between trophic 131

variability in southern African Barn owls Tyto alba and climate. Ostrich 70, 179-186. 132

Avery, D.M., 2004. Size variation in the common molerat Cryptomys hottentotus from 133

southern Africa and its potential for palaeoenvironmental reconstruction. Journal of 134

Archaeological Science 31, 273-282. 135

Chase, B.M., Chevalier, M., Boom, A., Carr, A.S., 2017. The dynamic relationship 136

between temperate and tropical circulation systems across South Africa since the last 137

glacial maximum. Quaternary Science Reviews 174, 54-62. 138

Chase, B.M., Faith, J.T., Mackay, A., Chevalier, M., Carr, A.S., Boom, A., Lim, S., 139

Reimer, P.J., 2018. Climatic controls on Later Stone Age human adaptation in Africa's 140

southern Cape. Journal of Human Evolution 114, 35-44. 141

Chase, B.M., Meadows, M.E., 2007. Late Quaternary dynamics of southern Africa's 142

winter rainfall zone. Earth-Science Reviews 84, 103-138. 143

Chevalier, M., Chase, B.M., 2016. Determining the drivers of long-term aridity variability: 144

a southern African case study. Journal of Quaternary Science 31, 143-151. 145

Deacon, H.J., Deacon, J., Scholtz, A., Thackeray, J.F., Brink, J.S., 1984. Correlation of 146

palaeoenvironmental data from the Late Pleistocene and Holocene deposits at 147

Boomplaas Cave, southern Cape, in: Vogel, J.C. (Ed.), Late Cainozoic Palaeoclimates 148

of the Southern Hemisphere. Balkema, Rotterdam, pp. 339-351. 149

Deacon, J., Lancaster, N., 1988. Late Quaternary Palaeoenvironments of Southern 150

Africa. Oxford University Press, New York. 151

Engelbrecht, F.A., Marean, C.W., Cowling, R., Potts, A.J., Engelbrecht, C., Nkoana, R., 152

O'Neill, D., Fisher, E., Shook, E., Franklin, J., Neumann, F., Scott, L., Thacther, M., 153

McGregor, J., Van der Merwe, J., Dedekind, Z., Difford, M., 2019. Downscaling Last 154

Glacial Maximum climate over southern Africa. Quaternary Science Reviews 226, 155

105879. 156

Faith, J.T., 2013a. Taphonomic and paleoecological change in the large mammal 157

sequence from Boomplaas Cave, Western Cape, South Africa. Journal of Human 158

Evolution 65, 715-730. 159

Faith, J.T., 2013b. Ungulate diversity and precipitation history since the Last Glacial 160

Maximum in the Western Cape, South Africa. Quaternary Science Reviews 68, 191-161

199. 162

Faith, J.T., Chase, B.M., Avery, D.M., 2019. Late Quaternary micromammals and the 163

precipitation history of the southern Cape, South Africa. Quaternary Research 91, 848-164

860. 165

Faith, J.T., Lyman, R.L., 2019. Paleozoology and Paleoenvironments: Fundamentals, 166

Assumptions, Techniques. Cambridge University Press, Cambridge. 167

Talma, A.S., Vogel, J.C., 1992. Late Quaternary palaeotemperatures derived from a 168

speleothem from Cango Caves, Cape Province, South Africa. Quaternary Research 37, 169

203-213. 170

Thackeray, J.F., 1987. Late Quaternary environmental changes inferred from small 171

mammalian fauna, southern Africa. Climatic Change 10, 285-305. 172

Thackeray, J.F., 2020. Late Quaternary micromammals and the precipitation history of 173

the southern Cape, South Africa - comment to the published paper by Faith et al., 174

Quaternary Research 91 (2019), 848-860. Quaternary Research.

UNEP, 1997. World atlas of desertification, 2nd ed. United Nations Environment 176

Programme, London. 177

Vogel, J.C., 1983. Isotopic evidence for the past climates and vegetation of southern 178

Africa. Bothalia 14, 391-394. 179